A typical damper system for a bridge based on the prior art, which is generally called a viscous damper, is shown in FIG. 1, and in this figure, the numerals 1 and 2 indicate a superstructure and a substructure respectively, and a cylinder 4 filled with a viscous fluid and set in a horizontal direction in a bracket 3 mounted on the superstructure is pivotally supported by a supporting shaft 5, and a tip of the piston lever 6 is pivotally supported by a supporting shaft 7 on the substructure. The viscous damper as described above does not resist to slow movements such as expansion or contraction of the superstructure due to temperature changes or for other reasons, but works as a viscous damper stopper which performs the virtually same role as a fixed support to quick vibrations as those in an earthquake. So in an earthquake, inertial force of the superstructure 1 is distributed to each of the substructures 2 to improve stability of the bridge in an earthquake. Generally, a damping coefficient of the viscous damper as described above is set to a large value exceeding critical damping coefficient.
Furthermore, the viscous damper as described above is used not as a stopper with a small damping coefficient which distribute inertial force, but as an energy absorber which positively absorbs vibration energy of the superstructure 1 for reducing vibration of the superstructure 1 by improving the damping characteristics of the entire structure and also reducing the earthquake force delivered to the substructure 2.
The viscous damper described above distributes inertial force of the superstructure 1 in an earthquake and does not reduce the inertial force of the superstructure 1, while a viscous damper as an energy absorber tries to reduce inertial force generated in an earthquake with its improved damping characteristics and does not try to distribute the inertial force. A damping system having both the function to reduce the inertial force and that to distribute inertial force as described above is required especially for installation in a bridge. However, such a device is not available, so it is necessary to install individual dampers each having the respective function in parallel. However, as a wide space to install a plurality of damping systems each having one of these functions is not available on the top surface of the substructure 2 of a bridge, generally only one unit having either one function is installed sacrificing another function, or a wider space is secured by additional cost to install a plurality of the units, which is a problem to be solved.
This invention was made to solve this problem in the conventional type of the damping system as described above, and its object is to provide a low construction cost variable damper system for a bridge which can achieve the same effects as those realized by a plurality of the conventional units each having an individual function because each unit has functions provided by a viscous damper, an energy absorber, and a stopper for preventing excessive response with a shock absorber and can be installed in a narrow space on the top of the substructure of a bridge without modifying it.